[43.15] The Spectacular Ionized ISM of NGC 55

Understanding the relationship between the star formation
process and the ISM is a key step towards unravelling the
complex history of galaxy evolution. A longstanding problem
has been the nature and importance of the feedback process
by which massive stars deposit energy into the ISM via
photoionization, stellar winds and supernovae. We have
obtained deep H\alpha, [SII] and [OII] images of the
edge--on SBm galaxy NGC 55, located in the nearby Sculptor
Group, which reveal a spectacular variety of ionized gas
features. NGC 55 is morphologically similar to the LMC,
but is slightly more luminous (M_B = -18.6). In
addition to its numerous bright HII regions,
NGC 55 hosts several large filamentary and bubble--like
features extending well above the main body of the galaxy,
reaching several kpc off the plane, as well as copious
amounts of truly diffuse ionized gas. We will discuss the
morphology and large scale distribution of both the
discrete star--forming regions and the filamentary/diffuse
ionized features, and investigate the possible
relationship between them. The current massive star
formation rate, as determined from the H\alpha emission
line flux, is derived and used to estimate the mechanical
luminosity from stellar winds and supernovae which is available to
drive the extra--planar ionized features. Emission--line
ratios ([SII]/H\alpha, [OII]/H\alpha) constrain the
ionization mechanism of the gas and we will
particulary focus our attention on those features located
far from bright HII regions. If these structures are
due to photoionization by Lyman continuum photons which
have leaked from discrete HII regions, then a correlation
is expected between the ionization state of a given feature
and the distance to the nearest HII region: we will
discuss the evidence for such an effect from our data.